Global warming is a serious problem to all human beings. In each country, to retard and stop the progress of global warming, energy saving, the use of new energy which does not emit CO2, and the like have been promoted.
In Japan also, to reduce CO2 emissions, the government determines the goal of new energy introduction, and promotes the introduction of electricity generation using new energy such as solar light, wind power, or the like, while also giving subsidy or the like.
Wind power electricity generation and solar electricity generation use natural energy, and have the advantage of no emission of CO2. However, because of their dependence on wind and weather, an electricity output generated thereby is not stable, resulting in concerns about an adverse effect on an electric power system and the deterioration of electric power quality. For the purpose of preventing such an adverse effect on an electric power system, when wind power electricity generation or solar electricity generation is introduced, it is requested to simultaneously introduce an accumulator system.
Concerning this point, a detailed description will be given. First, an electricity output generated by, e.g., wind power electricity generation significantly varies depending on wind conditions. If allowed to flow without any alteration, the generated electricity output exerts an adverse effect on the electric power quality of an electric power system. To prevent this, an accumulator system using a lead accumulator or the like produces an accumulator system output in accordance with the state of the electricity output generated by wind power. As a result, the electricity output generated by wind power and the accumulator system output are combined to allow a smoothed system output to flow to the electric power system, and enable avoidance of the adverse effect on the electric power quality.
Accordingly, a lead accumulator used for suppressing fluctuations in an electricity output generated by wind power is required to have a lifetime as long as that of wind power electricity generation equipment and lower cost. In this regard, the lifetime of the lead accumulator can be increased by ensuring proper operational conditions.
To thus ensure proper operational conditions and increase the lifetime of the accumulator, it is required to precisely recognize a remaining capacity as the present state of charge of the accumulator. If the state of charge can be precisely recognized, it is possible to increase the lifetime of the lead accumulator, and reduce cost (reduce the number of lead accumulators needed to suppress fluctuations in wind power output).
As a method for precisely recognizing the state of charge of a lead accumulator, a method disclosed in Patent Document 1 is known. In this method, a relational expression between the state of charge of an accumulator mounted in a vehicle, the terminal voltage and current thereof is obtained in advance, and the state of charge at the time of an idling stop is determined from the terminal voltage and current.
In Patent Document 2, a method is disclosed in which, when the state of charge of an accumulator is sensed, a determination method is changed such that, if the state of charge in a region in a high state of charge is sensed, it is determined from the relation between a discharging voltage and a discharging current or, if the state of charge in a region in a low state of charge is sensed, it is determined from the relation between a charging voltage and a charging current, and estimation accuracy is thereby improved.
In Patent Document 3 also, a method is disclosed in which a state of charge is detected from the relation between an average voltage and an average current during a given time.
Each of the conventional methods is a method in which a relational model between the state of charge of the accumulator, the terminal voltage and current thereof is produced in advance and, from the present measurement values of the terminal voltage and current, the present state of charge of the accumulator is estimated.